Hydrocarbons (oil, natural gas, etc.) may be obtained from a subterranean geologic formation (a “reservoir”) by drilling a well that penetrates the hydrocarbon-bearing formation. Well treatment methods often are used to increase hydrocarbon production by using a treatment fluid to interact with a subterranean formation in a manner that ultimately increases oil or gas flow from the formation to the wellbore for removal to the surface.
Recovery of hydrocarbons from a subterranean formation is known as “production.” One key parameter that influences the rate of production is the permeability of the formation along the flowpath that the hydrocarbon must travel to reach the wellbore. Sometimes, the formation rock has a naturally low permeability; other times, the permeability is reduced during, for instance, drilling the well. When a well is drilled, a drilling fluid is often circulated into the hole to contact the region of a drill bit. This drilling fluid can be lost by leaking into the formation. To prevent this, the drilling fluid is often intentionally modified so that a small amount of its liquid content leaks off and the remaining solid content forms a coating on the wellbore surface (often referred to as a “filtercake” or “mud cake”—described in more detail below). Once drilling is complete, and production is desired, this coating or filtercake must be removed to re-establish the flowpath from the formation into the well.
Well treatment fluids, particularly those used in fracturing (fracturing fluids) or those used in gravel packing operations (gravel packing fluids), may comprise a water or oil based fluid incorporating a thickening agent, normally a polymeric material. Polymeric thickening agents for use in such fluids may comprise galactomannan gums, such as guar and substituted guars such as hydroxypropyl guar and carboxymethylhydroxypropyl guar (CMHPG). Cellulosic polymers such as carboxymethyl cellulose (CMC) may also be used, as well as synthetic polymers such as polyacrylamide. Such fracturing fluids can have a high viscosity during a treatment to develop a desired fracture geometry and/or to carry proppant into a fracture with sufficient resistance to settling. These fluids can also develop a filter cake which includes the polymeric additives.
The recovery of the fracturing fluid is achieved by reducing the viscosity of the fluid such that the fluid flows naturally through the proppant pack. Chemical reagents, such as oxidizers, chelants, acids and enzymes may be employed to break the polymer networks to reduce their viscosity. These materials are commonly referred to as “breakers” or “breaking agents.” Such conventional fracturing fluid breaking technologies are known and work well at relatively low and high temperatures.
Hydraulic fracturing and gravel packing require the use of viscosified fluids to suspend or transport the gravel or proppant. However, whenever polymeric viscosifiers are used some degree of formation damage is created which requires removal to optimize oil and gas production and recovery. Therefore breakers, such as enzymes, are frequently employed to reduce or remove the effects of formation damage.
Most polymeric fluids used in oilfield applications damage the formation by leaving behind a filtercake used to control fluid leak-off into the formation and to restrict the inflow of reservoir fluids into the formation rock during drilling and completion techniques. If the filtercake damage is not removed prior to or during completion of the well, a range of issues can arise, for example, completion equipment failures, impaired reservoir productivity, and so on.
The major components typically found in filtercakes can include polymers, such as starch, guar, derivatized guars such as CMHPG, cellulosic polymers such as CMC, xanthan gum, polyacrylamides and co- or ter-polymers containing acrylamide, acrylic acid, vinyl pyrrolidone or acrylamido-methyl-propane sulfonate monomers and solids, such as carbonates, silica, mica and other inorganic salts and clays. The solids in the mud or fluid are sized such that they can form an efficient bridge across the pores of the formation rock as the well is being drilled or during injection of the fluid during the fracturing process. As the solids in the mud or fluid develop bridges across the exposed pores or pore throats of the reservoir, the polymeric fluid loss material from the mud or fluid can be co-deposited within the interstices of the solid bridging particles, thus sealing off the reservoir from the wellbore or fracture. These polymeric materials can comprise an integral component of the resulting filtercake, typically 17 to 20 weight percent of the dry filtercake, and can be responsible for the ultra-low permeability of the filtercake.
Cleanup of polymer-based filter cakes in long horizontal and multilateral wells is a difficult, but very important task. Both mechanical approaches such as water jetting, and chemical means such as acids, oxidizers, and enzymes, have been used in the field with limited success to remove mud cakes. Conventional chemical treatments for removing filtercake from the fracture typically involve placing aqueous breakers into the fracturing fluid that forms the filtercake, wherein the breaking action is delayed by temperature or encapsulation. For mud cake removal after drilling, conventional chemical treatments involve placement of breaker solutions near the filter cake with a soak time to a allow reaction to occur. These treatments may use oxidizers, enzymes or a combination, and in the case of mud cakes, may also employ mineral and/or organic acids, or chelating agents. Generally, the oxidizer or enzyme breakers digest the polymer layer in the filtercake, and when the solids in the mud cake are soluble such as carbonate, the chelants and acids dissolve the solid portion of the filtercake. These methods have serious limitations, which can adversely affect well performance. Acids and oxidizers are non-specific, and are very reactive, because of which uniform removal of the filter or mud cake is very difficult.
As compared to oxidative breakers, benefits potentially associated with enzymes include high selectivity towards the polymer backbone, autocatalysis which means just small amounts of the enzyme breaker, can be effective, and a better health, safety and environmental (HSE) profile. Enzymes can be higher in molecular weight than oxidative breakers so that they tend not to leak off into the surrounding formation, and can also be less susceptible to dramatic changes in activity by trace contaminants. Enzymes can be used to degrade polymers and can facilitate uniform treatment of the filter cake induced damage. For example, well treatment fluids for gravel packing, available under the trade designation MudSOLV and described in U.S. Pat. No. 6,638,896 and U.S. Pat. No. 6,140,277, the disclosures of which are incorporated by reference herein in their entirety, describe a gravel carrying fluid containing enzyme for polymer removal in filter cake remediation, chelating agent to dissolve carbonate, and a cationic or non-ionic viscoelastic surfactant (VES) system at a sufficiently high concentration to viscosify the fluid.
However, enzymes used in conventional filter cake removal are subject to some limitations, such as the loss of suitable enzymatic activity at downhole conditions and possible permanent denaturation of the enzyme, rendering its activity to be essentially zero, before a sufficient period of time has elapsed that is adequate for the enzyme to break the polymer. For oilfield applications, enzyme reaction times are usually at least 4 hours at temperature for mud cake removal and even longer for fracture cleanup. Activity of the enzyme, or the ability of the enzyme to catalyze breaking of the polymer by hydrolysis, for example, may also be an important benefit. However, because the enzyme is a catalyst rather than a reactant which would otherwise be consumed in the breaking reaction, a small amount of active enzyme may be effective where the enzyme concentration is not rate-limiting.
Other limitations of enzymes include these materials being extremely sensitive to pH, ionic strength and temperature. High salinity, especially in the presence of divalent ions like calcium, can also prematurely inactivate and/or coagulate enzymes.
Enzymes begin to lose their activity at higher temperatures. A major limitation of enzymes is their inability to stay active at temperatures above 93° C. (200° F.). For example, experimental studies reported in the literature show that the activity of enzymes at 97° C. (207° F.) is less than 10% of activity at 93° C. (200° F.). There can be variations in their activity at the upper temperature limit depending on the source of the enzyme, as one hemi-cellulase still retains some activity at 135° C. (275° F.).
For an improved enzyme breaker, oilfield applications generally seek applicability across a broader pH, salinity, and temperature range, e.g. above 93° C. (200° F.), above 107° C. (225° F.), or even above 121° C. (250° F.); efficacy at pH levels above 10, above 10.5, above 11 or above 11.5, storability without refrigeration, e.g. at or above ambient temperature; improved logistics; and easy mixing.
The temperature dependence of enzymes must be understood to apply them correctly at oilfield conditions. Within its activity range, an enzyme generally speeds up a reaction more as the temperature is increased. However, reactions that deactivate the enzyme, e.g. the denaturation of proteins, are also favored at higher temperatures. Hence, there exists an optimum temperature for a given time and turnover requirement. The temperature limit for use of a given enzyme depends critically on the kinetics of turnover, deactivation, and transport. The faster an enzyme can be brought to the place where it has to do its job, the higher the maximum temperature at which it can be used. Wells with higher bottomhole static temperature (BHST) may still be treated, depending on the temperature gradient and the process design. In these cases, field treatment procedures must be tuned to bring enzyme to the desired cleanup location downhole at a sufficiently low temperature and for a sufficient time to enable it to degrade the polymer, before the enzyme is deactivated and/or coagulated by the heat.